Manufacture of bessemer steel having reduced strain sensitivity



Dec. 18, 1951 H. K. WORK ETAL MANUFACTURE OF BESSEMER STEEL HAVING REDUCED STRAIN SENSITIVITY 2 SHEETS-SI-IEET 1 Filed June 4, 1949 INVENTORS Hero/0%. Work amp/mama Websfer 66-744, W A

Dec. 18, 1951 H. K. WORK ETAL 2,578,372

MANUFACTURE OF BESSEMER STEEL HAVING REDUCED STRAIN SENSITIVITY Filed June 4, 1949 Q 2 SHEETS-SHEET 2 /OO 5 a0 S 3 60 Q K Y K U 85 Q a Q o a 0O 2 4 6 6 IO PE/ecE/vrCbw Wop/r fig. 5. fig. 4.

PERCENTCOLDWORK ERcEA/TCDLDWORK /zo0 IMPACFFOOTPOU/VOS INVENTORS Haro/oK Work v and/Q/c/ra/d/Q Websfer Patented Dec. 18, 1951 UNITED STATES PATENT OFFICE MANUFACTURE OF BESS'EME-R STEEL HAV- ING REDUCED STRAIN SENSITIVITY 1 Claim.

This invention relates to a process of makin Bessemer steel havin reduced strain sensitivity and having other properties comparable to steels produced by conventional open hearth process. This application is in part a continuation of our copending application Serial No. 706,230, filed October 28, 1946, now abandoned,

It is well known to .metallurgists that when steel is worked below its temperature of recrysta1lizationtermed cold working--by' any of the well-known methods such as hammering, drawing, bending, rolling, et'cfl, its hardness is increased and its toughness, or resistance to impact, is decreased; This tendency to become hard and brittle as a result of cold working is denominated strain sensitivity? As in the case of other physical properties, this property or characteristic of steel is influenced by the chemical composition of the steel. Investigations have indicated that strain sensitivity is a property of ferrite, and so' is most pronounced inlow carbon steels in which ferrite is the predominant constituent. In higher carbon steels strain sensitivity is less apparent, and in steels of eutectoid or hypereutectoid composition this characteristic disappears. It has, however, beenvery generally observed that steels having. practically identical hypoeutectoid compositions, both with respect to alloying metals and metalloids, and displaying a close similarity in other physical properties may exhibit widely different strain sensitivity characteristics. This may be true of two supposedly identical steels made by identical practices.

One method of controlling the strain sensitivity of steel is disclosed in Graham and CaseP'atent 2,174,740, according to which aluminum is used in considerable quantity to produce strain insensitive steel. However, for many purposes it is desirable to keep the aluminum additions to a minimum. For example, where it is desired to make rimming or efiervescing steel, it is impossible to add the amount or aluminum taught by Graham and Case without also deoxidizing the steel to such an extent that rimming will not take place. The Graham and Case methodis not applicable to capped, rimmed, semi-killed or silicon' killed steels which comprise a large proportion of steel manufacture.

Almost without exception, steels made by conventional bottom blowing Bessemer practice are more strain sensitive than steels made by other practices. Because of this greater. strain sensitivity, conventional Bessemer steel isunsuitedto a large number. of applications which. conventional. open. hearth. steel satisfies. That this 2 handicap is serious may be observed from the unremittmg' decline in relative production of Bessemer steel for the last twenty'years or more.

' By our invention, we are able to make steel by a duplex process of bessemerizing and dephosphorizing, which is comparable in all physical properties, including strain sensitivity, to that made by conventional open hearth practice. Since a Bessemer blow is completed in a matter of about twenty minutes and the dephosphorizing step a little more time, the process of our in vention is several times faster than conventional open hearth practice, which may require'twelve hours for the production of one heat. For this and other reasons, our process is cheaper than conventional open hearth refining- In the accompanying drawings which illustrate a preferred manner of carrying out our invention Figure 1 is a vertical section through a side blown converter; and

Figures 2, 3 and 4' are graphs showing the strain-sensitivity characteristics of steels of various compositions and made by various steel-making practices.

In carrying out our invention, molten iron of conventional composition suitable for acid Bessemer practice'is poured into an acid-lined converter and blown until the silicon, manganese and most of the carbon are oxidized. During thebldwing step the conditions are controlled so that at the end. of the blow' the blown metal does not contain over about 0.008% nitrogen. While this result may be accomplished in various ways, one'suitable method is to employ a side blown converter such as is shown in Figure 1 and to operate it in the manner now to be described. We have found that" in order to produce a low nitrogen content in the blown. metal, the streams of air from the'tuyer'es 2 s'h'oul d be directed into the" bath 3 at an angle. 4 with the bath surface 5' of about 4 to l5.. We have found that if the angle of' inclination 4'. is 'gre'a'ted than about 15, it" is impossible to obtain blown metal having the nitrogen content characteristic of our invention. On the other hand, we have found that there is a minimum angle of inclination of about 4"be1'ow which the blow becomes difficult to control because the bath surface is not a smooth plane' but' a bubbling or boiling surface of considerable irregularity. Ifthe angle 4 is too small, the bath agitation causes the air blast toflow entirely over the bath surface at one instant and partly or wholly within the bath at'another, and operationis erratic. Q

Another factor in producing blown metal containing not over about 0.008% nitrogen is the location of the tuyeres 2 with respect to the bath surface. In general, it is ineiiicient to blow with the tuyere mouths. 6 any substantial distance above the bath surface. While it ispossible to have the tuyere mouths somewhat below the bath surface, the farther below the bath surface they are, the greater is the tendency to pick up nitrogen. We therefore prefer to locate the tuyere mouths substantially at the bath surface and to direct the blasts ofair from the tuyeres into the bath at an angle of about 4 to with the bath surface inorder to produce blown metal having a low nitrogen content.

We employ for carrying out our process a tiltable acid-lined side blown Bessemer converter whose tuyeres all have approximately parallel axes. The converter shown in Figure 1 is such a converter. It has the general form of a cylinder whose axis is horizontal and perpendicular to the plane of the paper viewing Figure 1 except that an open nose portion is provided at the upper part of the converter as it is shown in Figure 1. All of the tuyeres 2 are located along one side of the converter with their axes approximately parallel to each other and in a plane parallel to the horizontal axis of the converter so that the axes of all of the tuyres make approximately the same angle with the general plane of the surface of the molten metal in the converter.

When a converter of the type above mentioned is employed the positions of the tuyere mouths relative to the metal surface and the angle which the axes of the tuyres make with the general plane of the metal surface may easily be adjusted by tilting the converter about its horizontal axis.

An approximately known quantity ofmolten ferrous metal is introduced into the converter so that when the converter is positioned with the axes of the tuyeres disposed at an angle with the general plane of the metal surface of between about four and about fifteen degrees the mouths of the tuyeres are located approximately at the metal surface. However, it is not necessary that a precisely measured quantity of molten ferrous metal be introduced as small irregularities in the quantity of metal introduced may be compensated for by tilting the converter slightly one way or the other to raise or lower the tuyere mouths to position them approximately at the metal surface while the angle between the axes of the tuyeres and the general plane of the metal surface is within the limits of about four and about fifteen degrees.

For any given converter the approximate quantity of molten ferrous metal which should be introduced may be initially determined experimentally as it depends upon the internal size and shape of the converter. Once the approximate quantity is determined the converter charge may be roughly measured by weight, volumetric measurement or otherwise so that an approximately known quantity of molten ferrous metal is introduced for each blow. As the converter lining wears down in service the quantity of molten ferrous metal which should be charged for each blow will increase somewhat but the increase from blow to blow is small and may be easily determined by observation.

By controlling the position and angle of inclination of the tuyres with respect to the bath surface as above specified, a higher bath temperature is obtained than in conventional bottom blowing practice and the nitrogen content of the blown metal is consistently not over about .008 indeed consistently well below that figure. The following analyses of blown metal before dephosphorization are typical of the results obtained by our process:

Blow 0 Mn P s s1 N 515 5- As the above table shows, the temperature of the blow may be as high as well above 3100 F. without adversely affecting the nitrogen content of the blown metal, as blows A and F, the hottest and coldest respectively, both have the same nitrogen content. In conventional Bessemer practice the temperature of the blow must be maintained not over about 2950" F. to keep the nitrogen content not over about .008%.

While we do not wish to be bound to any theory, we believe that our process produces steel which is less strain sensitive than conventional Bessemer steel because of the following considerations: In the case of aluminum free steels, we believe that some compound of iron, nitrogen and phosphorus, perhaps iron nitride dissolved in iron phosphide, is responsible for strain sensitivity. Such a compound could easily be formed in the course of conventional bottom blown Bessemer blowing since the air blast introduces large quantities of nitrogen into the molten iron which already contains phosphorus. However, as has been mentioned, a higher bath temperature is attained in our side blowing practice and other conditions within the bath may be different. The higher temperature may be unfavorable to formation of the assumed iron-nitrogen-phosphorus compound, or an unstable compound may be formed which breaks down during subsequent treatment. We have found in analyzing a substantial number of heats that the nitrogen as determined by conventional methods of chemical analysis was lower in our steels than in conventional bottom blown Bessemer steels. In our steels the nitrogen was as low as 0.004% and seldom exceeded 0.008%, whereas in conventional Bessemer steels the nitrogen ranges from about 0.012% to as high as about 0.025%.

After the charge of molten iron has been blown in the manner described to reduce its carbon content to the desired extent and to produce blown metal containing not over about 0.008% nitrogen, the blown metal, as free from slag as possible, is poured into a, transfer ladle and taken to a basic lined dephosphorizing vessel. A mixture of lime and oxide, i. e., limestone and roll scale, having a preferred ratio of about two parts lime to one part iron oxide, or other known dephosphorizing compound, is charged into the dephosphorizing vessel before or with the blown metal. The highly basic slag thus formed reacts with the blown metal to remove the phosphorus in a few minutes, after which additions may be made as desired, and the heat tapped.

Thevessel or furnace for dephosphorization may be any basic lined furnace such as an open hearth furnace, an active mixer, 31. Brackelsburg furnace or an electric furnace, or, if the Bessemer blow is hot enough, a vessel unprovided with heating means. It is very convenient to use a tilting open hearth furnace of conventional design. The term vessel used hereafter in this connection will be understood to include any suitable type of basic lined dephosphorizing vessel or furnace. The dephosphorizing vessel may be used for further refining if desired and since the converter may be of considerably smaller capacity than the open hearth furnace or other dephosphorizing vessel, several blows may be combined in the dephosphorizing vessel and dephosphorized together. After the heat has been tapped from the dephosphorizing vessel, the slag may be retained in the vessel and re-used to dephosphorize fresh charges from the converter. The slag may be re-used in this manner until its dephosphorizing power is exhausted, or its dephosphorizing capacity may be maintained by adding a further quantity of lime each time a new charge of blown metal is introduced. This practice is economical of time, heat, and materials, and when the slag is finally withdrawn, its high phosphorus content renders it valuable as an agricultural fertilizer.

Figures 2, 3 and 4 illustrate the desirable strainsensitivity characteristics of steels made by the method of our invention. All comparisons are between steels of strictly comparable static physical properties and chemical analyses, except where noted otherwise. The strain-sensitivity characteristics were obtained in the manner described in the paper by H. W. Graham and H. K. Work, A Work-Brittleness Test for Steel published in the Transactions of the American Society for Testing Materials, vol. 39, p. 571, 1939.

In Figure 2 are shown strain-sensitivity characteristics of three carbon steels, as follows:

(1) Steel made according to our invention,

(2) 'Conventional bottom blown Bessemer steel which has been dephosphorized,

(3) Conventional open hearth steel.

The analyses of these steels were as follows:

It is obvious from Figure 2 that the embrlttling effect of cold work is very little greater in steel (1), made according to our invention, than in steel (3), conventional open hearth steel, and is much less than in conventional Bessemer steel (2). The only significant difference in chemical composition is seen to be in the nitrogen contents, the steel of our invention having a nitrogen content only slightly higher than that of conventional open hearth steel, but appreciably lower than that of conventional Bessemer steel. We believe this lower nitrogen content is responsible in some way for the superior properties of our steel.

Figures 3 and 4 show that it is not necessary in the practice of our invention to dephosphorize the steel to the level of conventional open hearth steel to obtain good strain-sensitivity properties. In Figure 3 impact strength against cold work is plotted for a series of steels representative of our process in which the blown metal has been dephosphorized to the values indicated on the curves. In Figure 4 the same data are plotted for steels representative of conventional Besseiner practice, similarly dephosphorized. Conventional Bessemer steel with a phosphorus content of 020% which represents highly efiicient dephosphorization, is shown to become embrittled more rapidly than the steel of our process having 081% phosphorusa value attainable with very rudimentary dephosphorization practice.

From the foregoing it may be seen that our invention makes possible the manufacture by the Bessemer process, followed by dephosphorization, of steel having strain sensitivity as well as all other physical properties practically equal to that of steel made by the conventional open hearth process. Our process, however, is faster and cheaper than conventional open hearth refining.

The invention is not limited to the preferred embodiments which have been given merely for purposes of illustration but may be otherwise embodied or practiced within the scope of the following claim.

We claim:

The process of making Bessemer steel having reduced strain sensitivity which comprises introducing an approximately known quantity of molten ferrous metal into a tiltable acid-lined side blown Bessemer converter whose tuyeres all have approximately parallel axes, tilting the converter until the mouths of the tuyres are located approximately at the metal surface and the axes of the tuyeres make an angle with the general plane of the metal surface of between about four and about fifteen degrees with the tuyeres tilted downwardly toward their mouths, blowing air into the metal through the tuyres to refine the metal while maintaining the nitrogen content of the metal not over about 008%, removing the acid slag and reacting the blown metal with a basic slag to remove phosphorus.

HAROLD K. WORK. RICHARD R. WEBSTER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 217,962 Thomas July 29, 1879 278,738 Reese June 5, 1883 407,117 Robert July 16, 1889 423,612 Ludlow Mar. 18, 1890 481,881 Witherow Aug. 30, 1892 511,919 Tropenas Jan. 2, 1894 698,431 Behrend Apr. 29, 1902 698,787 Behrend Apr. 29, 1902 986,534 Woods Mar. 14, 1911 1,140,550 Weissenburger May 25, 1915 1,899,161 Kuzell Feb. 28, 1933 FOREIGN PATENTS Number Country Date 885,968 France June 15, 1943 887,079 France July 26, 1943 OTHER REFERENCES The Iron Age, March 22, 1945, pages 61 and 62. Published by the Chilton Co., Philadelphia, Pa.

Stahl und Eisen, vol. 68, October 7, 1948, pages 387 to 395. 

